The present application is directed to lighting devices, and more particularly to ballast circuitry for discharge lamps. Current fed bipolar junction transistor (BJT) based inverter ballasts are widely used in the lamp-lighting industry due to their inherent parallel lamp operation and output transformer isolation features. Providing transformer isolation permits parallel lamp operation and re-lamping of the lighting system to take place without requiring the shutdown of the power inverter of the entire system. Therefore, a lamp failure in the system can be replaced when it is needed while the remaining lamps are maintained in an “on” state. This therefore also reduces the maintenance and operational costs of such systems.
An example of a current fed inverter ballast having an instant program start configuration for use with parallel lamps has been described in U.S. Pat. No. 7,193,368, titled Parallel Lamps With Instant Program start Electronic Ballast, to Chen et al., issued Mar. 20, 2007. This ballast takes advantage of the beneficial aspects of a program start ballast (e.g., longer lamp life) and combines it with the advantages of an instant start ballast (e.g., quick start time) to produce an improved lamp ballast wherein parallel lamps are driven. Another circuit of this type is set forth in U.S. application Ser. No. 11/645,939, titled Switching Control For Inverter Startup And Shutdown, to Chen et al. filed Dec. 27, 2006, which describes a current fed BJT based inverter including a low cost shutdown circuit. Both U.S. Pat. No. 7,193,368 to Chen et al., and U.S. application Ser. No. 11/645,939 to Chen et al. are both hereby incorporated by reference in their entireties.
A drawback of existing current fed BJT based ballast systems which provide output transformer isolation is that they tend to have an efficiency which is relatively low compared to non-isolated lamp lighting ballasts due to the isolation transformer and operation mode of the BJTs. Therefore, a particular issue with such BJT based electronic ballasts has to do with the optimization of their base drive to improve the operational efficiency of these devices. Attempts to optimize the base drive signals commonly results in overdriving of the base-to-emitter junction of the BJT switches. This is a particular issue where the base of the BJT is driven by a parallel diode-resistor arrangement. In such configurations, when the base-to-emitter junction is overdriven, an undesirable increase in power dissipation takes place in the BJTs, and a higher circulating current exists in the ballast resulting in lower ballast efficiency. Another drawback which occurs due to overdriving is that dead-time, i.e., the overlap between the two transistor switching times, increases, leading to a higher current crest factor. Where current crest factor is the peak current divided by the root-mean-square (rms) current of lamp. ANSI standards require current crest factor to be less than 1.7.
Further, when current fed BJTs are used in conjunction with high efficiency lamp striations are known to occur even at room temperature. Striations manifest themselves as dark bands along the length of lamps and are particularly prevalent in lamps which use a high percentage of Krypton (Kr), which is employed as a buffer gas to improve the efficacy and usefulness of the lamps. For example, high efficiency lamps, may have a content of approximately 40 percent to 70 percent of Krypton (Kr).
Concepts of the present application are intended to address these and other outstanding issues as they relate to current fed BJT based inverter ballasts.
Prior art which may be of interest to the above-identified issues and others include U.S. Pat. No. 4,682,082, titled Gas Discharge Lamp Energization Circuit, to MacAskill et al., issued on Jul. 21, 1987; U.S. Patent Application Publication No. US2006/0103328, titled Striation Control For Current Fed Electronic Ballast, to Chen et al., published on May 18, 2006; U.S. Pat. No. 6,465,972, titled Electronic Elimination of Striations In Linear Lamps, to Kachmarik et al., issued on Oct. 15, 2002; and WO2006/051459, titled ANTI-STRIATION CIRCUIT FOR A GAS DISCHARGE LAMP BALLAST, to Fang, published May 18, 2006.